This project involves the development of a system to measure PO2 and PCO2 in extracorporeal circuits for more than 24 hour periods without blood loss. Picomole amounts of O2 and CO2 are extracted from the circulation with a special probe. Oxygen is analyzed by amperometry and CO2 is measured by calorimetry of the reaction of CO2 with LiOh. Tests have shown the system to be stable within 5% over 25 hour periods.